1. Field of the Invention
The present invention relates to a diffractive optical element for producing diffracted light flux relative to an incident light, and more particularly to a diffractive optical element constructed by laminating a plurality of diffractive elements.
2. Description of Related Art
A diffractive optical element is an optical element having lattice structure of slits or grooves spacing at even intervals with several hundreds lines per a small distance (about 1 mm), and it has characteristic that when a light is incident to it, it produces diffracted light flux in a direction determined by the wavelength of the light and the separation (pitch) of slits or grooves. Diffractive optical elements like this are used in various kinds of optical systems, for example, an optical element/used as a lens for converging a specific order of diffracted light into a point is known.
In diffractive optical elements like this, a diffractive optical element called a plurality of layers type has been proposed. The diffractive optical element of this type has structure laminating a plurality of diffractive elements having a saw-tooth shape surface in a form appressed or separated with each other. It has a characteristic having high diffractive efficiency over almost entire range of a required wide wavelength range (for example, whole visible light range), in other words, good spectral characteristic.
As shown in FIG. 9, structure of a laminated type diffractive optical element is generally composed of a first diffractive element 310 made of a first material, and a second diffractive element 320 made of a second material having different refractive index and dispersion from those of the first material. The faces of respective diffractive elements facing each other are saw-tooth shape surfaces as shown in the drawing. Here, in order to satisfy the condition for correcting chromatic aberration at predetermined two wavelengths, the groove height d1 of the first diffractive element 310 is set to a predetermined value and that d2 of the second diffractive element 320 is set to another predetermined value. Accordingly, diffraction efficiencies regarding the predetermined two wavelengths become 1.0 and considerably high diffractive efficiency can be obtained at the other wavelength. In the transparent type diffractive optical element, diffractive efficiency is defined as a ratio ηA (=A1/A0) of amplitude of a first order diffracted light A1 to that of an incident light A0. Alternatively, diffractive efficiency is defined as a ratio ηI (=I1/I0) of intensity of a first order diffracted light I1 to that of an incident light I0. By the way, the square of light amplitude generally indicates light intensity (for example, A12=I1). In this case, light loss caused by absorption or scattering is supposed not to exist.
However, in a conventional diffractive optical element, in order to satisfy the condition for correcting chromatic aberration at predetermined two wavelengths, the groove height d1 of the first diffractive element 310 and that d2 of the second diffractive element 320 become considerably larger relative to the case each diffractive element is used independently. The groove height of a universally well-known diffractive optical element (a single layer diffractive optical element) is about 1 μm. On the other hand, in almost all these multi-layer type diffractive optical elements, the optimum designed value of the groove height of a diffractive optical element, in other words, the total groove height D (=d1+d2) of all elements becomes more than 10 μm.
In the case of the groove height of whole diffractive optical element becomes large, even if an incident light slightly tilts from the reference optical axis, light flux properly passing through both diffractive optical elements 310 and 320 decreases. Accordingly, it has been a problem that when a rate of decrease in diffractive efficiency relative to variation in the incident angle of the incident light (hereinafter called angular characteristic) is applied, the angular characteristic of the multi-layer type diffractive optical element decreases largely relative to an ordinary diffractive optical element. Moreover, it has become difficult to form grooves of the diffractive element in accordance with the width of the pitch thereof.
Furthermore, in a conventional multi-layer type diffractive optical element shown in FIG. 9, since the groove height d1 of the first diffractive element 310 differs from that d2 of the second diffractive element 320, the diffractive elements 310 and 320 have to be made separately with the same procedure. Finally, both diffractive elements 310 and 320 must be precisely positioned, so that it becomes difficult to manufacture.